The use of insulating materials such as, for example, plastic foams and glass wool for the comprehensive thermal insulation of buildings is generally known in numerous configurations. In one of the configurations most frequently employed, panels of the insulating material having a thinness of about 4 cm to 40 cm are attached in or on wall constructions or else under the roof between, underneath or on the rafters.
However, the insulating materials only exercise their full effect if they prevent air exchange through them since otherwise, considerable amounts of heat would be lost with the air that diffuses from the heated interior to the outside. This situation would be reversed in the summer, when the warm air penetrates inside, heating up the building in an unwanted manner, with the result that an air conditioning system that might be present would consume considerably more energy.
Consequently, at first it seemed to be an obvious approach to seal the thermal insulating materials with a material that is impermeable to air, for example, a sturdy polyethylene or PVC film, but this did not bring about the desired result because it is practically impossible to avoid the occurrence of damage such as cracks or holes, so that air nevertheless manages to get into the insulating material. When the air cools down, the entrained moisture condenses out, so that over the course of time, there are considerable water accumulations which can practically no longer be eliminated by drying. In the meantime, the water not only causes corrosive damage but it also reduces the effect of the thermal insulation.
As is like wise common knowledge, preference is thus given to those sealing materials that counteract but do not entirely prevent diffusion of the air and of the water vapor contained in it, so as to allow a reverse diffusion of the water vapor, in other words, drying of the insulating material.
Such materials, which are referred to as vapor retarders and which are normally employed in the form of films or layered materials, are polymers such as polyethylene, polyamides, ethylene acrylic acid copolymers and polyesters in a thin layer. More information on this can be found, from example, in the patent specifications DE-A 195 14 420 and 199 02 102.
The vapor retarders made of these materials, in the form of a film or of a suitable layered material, usually entail the advantage that their resistance against the diffusion of the water vapor is dependent on the relative humidity. If the relative humidity is low, this resistance is higher than when the relative humidity is height. This facilitates the dry of the damp insulating materials in the summer air, which is usually humid. For instance, the water vapor diffusion resistance value (sd value) according to German standard DIN 52165 of a 60-μm thick polyamide film amounts to 4.5 m at 3% relative humidity and only approximately 0.5 m at 80% relative humidity.
The sd value corresponds to the thickness of a stationary air layer whose diffusion resistance is as high as that of the specimen, that is to say, the thin polyamide-6 film in this example. This value is ascertained according to German standard DIN 52615, usually employing the dry cup method (dry area procedure), between the two humidity values of 0% and 50%, averaging 25%, and employing the wet cup method (damp area procedure), between the two humidity values of 50% and 95%, averaging 72.5%.